The present invention relates to the field of optoelectronics, in particular, to laser-diode units for generating a frequency-stabilized narrow-bandwidth light. The invention also relates to a method of generating a stabilized narrow-bandwidth light of a selected frequency. The invention may find application in various fields where short wavelength compact lasers are used, such as flat-panel displays, projection displays, optical data readers, optical sensors, laser measurement systems, optical data storage, etc.
At the present time, lasers find wide application in various fields. Especially in the recent years a tendency is observed for replacement of traditional solid-state and gas lasers with semiconductor lasers in view of their smaller dimensions, simplicity of use, and low cost. It is understood that such replacement is possible only if the replacement laser diodes possess at least the same properties with regard to the bandwidth characteristics and frequency stability as those in the lasers to be replaced. Semiconductor lasers with the above characteristics may find use in such fields as optical emission spectroscopy, laser-induced fluorescence analysis, transmission-absorption analysis, reflectometry, elipsometry, polarimetry, interferometry, Raman scattering analysis, non-linear optical diagnostics, etc.
It is obvious that in modern optical high-density data transmission systems stabilization of frequency is a very important factor for increasing density of information to be transmitted through the communication line. Therefore there is a great demand for semiconductor laser systems capable of generating a frequency-stabilized light of a selected wavelength in a narrow bandwidth of the light spectrum.
U.S. Pat. No. 5,812,716 issued in 1998 to Isamu Ohishi discloses an optical module and a temperature control method, wherein the optical module includes a light-emitting semiconductor laser, an optical waveguide in the form of a planar optical waveguide, a cooling device in the form of a Peltier device with a thermister for cooling the laser, and a package for containing the laser, one end portion of the optical waveguide, and the cooling device. The laser and the optical waveguide are optically coupled to each other. The optical waveguide has a grating section formed at the one end portion contained in the package for selectively reflecting only light with a fixed wavelength. The cooling device cools the grating section in addition to the light-emitting device. In this device, the grating section functions as a reflecting narrowband filter.
One advantage of the optical module of U.S. Pat. No. 5,812,716 is that it solves the problem of frequency stabilization by using a precision cooling system for stabilizing parameters of the system due to temperature stabilization. Another advantage is narrowing of the spectrum of the emitted light. Finally, the use of a Bragg grating section makes it possible to narrow the bandwidth.
However, in the optical module of U.S. Pat. No. 5,812,716 the Bragg grating is used as a passive frequency filter mainly for cutting off side modes of the semiconductor laser spectrum. As a result, the mean power of the output light emitted from the system is reduced proportionally to the cut-off portions of the spectrum.
U.S. Pat. Nos. 5,717,804 and 5,930,430 issued to J. J. Pan et al. in 1998 and in 1999, respectively, describe a method of narrowing the output wavelength and the laser diode assembly which fulfills the above function. The assembly contains a laser diode and an optical fiber section which receives the light output from the laser diode. The optical fiber section also has a fiber grating designed to operate as a bandpass filter. This fiber Bragg grating is designed so that the reflection band is partial, i.e., only a fraction of the light from the laser diode is reflected back. The reflected light increases the output of the laser diode at this wavelength band as compared to the light of other wavelength band of the light spectrum.
According to another embodiment of U.S. Pat. Nos. 5,717,804 and 5,930,430, two fiber (Bragg) gratings are used instead of one so that the combination of two gratings functions as a bandpass filter. As stated previously, a uniformly sinusoidal fiber Bragg grating has a precise and narrow wavelength band in which light is reflected by the fiber grating. Light outside the band is transmitted through the fiber grating. There are two techniques to widen the reflection band. The fiber grating may be linearly chirped, i.e., the periodicity of the fiber grating can be linearly varied. Alternatively, the fiber grating is created from a cascade of uniform fiber gratings, the reflection band of each uniform fiber grating contiguous with each other. The two fiber Bragg gratings have such a widened reflection range. Each grating has a slightly different wavelength range, as illustrated in FIG. 5 of the cited U.S. patents, which plots transmittance, the inverse of reflectance, versus wavelength for both gratings. The combination of the two gratings in series in the optical fiber section transmits light in a very narrow range (band), so that the optical fiber section now includes a high-performance bandpass filter. Light outside of the narrow range from the laser diode chip is reflected. Only light in the very narrow range passes out from the laser diode assembly.
However, as can be seen from the same FIG. 5 of the cited patents, the aforementioned laser assembly practically does not increase a signal/noise ratio and functions only with regard to narrowing of the bandwidth of the transmitted spectrum. It is an obvious disadvantage of the laser assembly of the aforementioned type which limits the scope of its practical application. Another disadvantage of the aforementioned known device is complicated manufacturing and assembling difficult for realization under industrial conditions.
It is an object of the invention to provide a laser-diode device, which is characterized by an increase in an output signal/noise ratio, increases the output light power at a selected narrow wavelength band, and provides a frequency stabilized narrow-band light of a selected wavelength at the output. Another object is to provide a laser-diode device which is suitable for manufacturing and assembling under industrial conditions. Still another object is to provide a method of stabilizing frequency and narrowing the linewidth of the spectrum of the light emitted from the laser device through external extended laser cavity.